The present invention relates to lead acid batteries with pasted positive and/or negative plates and a process for manufacture thereof.
The pasted positive plates in the lead acid batteries for stand-by service or in the portable lead acid batteries are divided, based upon the construction, into flat-plate type and clad type. In the clad type, a cylinder of active material is surrounded by an envelope in the form of a slotted tube so that a long life of a lead acid battery with the clad type pasted positive plates is ensured. On the other hand, the flat-plate type pasted positive plates are advantageous because of their simple production process and because of their unique discharge characteristics so that the lead acid batteries with the flat-type positive plates have been widely used for starting the engines of automobiles. Since heavy current is required to start an automobile engine, the batteries for automobiles must have a high output characteristic and at the same time must be inexpensive. Therefore, the batteries for automobiles are limited to the lead acid batteries with the flat-plate type pasted positive plates. Because of the low costs of the lead acid batteries with the flat-type pasted positive plates, they have been widely used for motive-power service, such as electric cars, golf carts, forklifts and so on.
The lead acid batteries with the flat-type pasted positive plates are also widely used as the source of power for television sets and tape recorders. In order to facilitate the handling of these batteries, the containers are totally closed to avoid the spill of an electrolyte or an electrolyte in the form of a gel is used. The flat-plate type lead acid batteries replace the clad-type lead acid batteries in some cases as stationary batteries. The lead acid batteries with flat-plate type pasted positive plates which have been widely used in various fields as described above, have their own problems depending upon their purposes. For instance, the lead acid batteries for starting automotive engines must have an excellent rapid discharge characteristic as well as a long life, and the batteries for driving electric cars must has a high energy density as well as a long life. Furthermore, for any lead acid batteries whatever their uses may be, they must be fabricated by a simple process so that their cost may be low.
A most common process for preparation of flat-plate type pasted positive plates comprises a step of pasting a paste consisting of an active material and diluted sulfuric acids to pockets of a grid made of a lead alloy, a step of drying and a step of forming.
Therefore, the improvements of flat-plate type pasted positive plates are dependent upon the composition of paste, materials and construction of grids, the improvement of the step for pasting the paste to the grids. Briefly stated, the more water and diluted sulfuric acid are added to an active material of lead powder, the higher the efficiency becomes, but the shorter the life becomes.
As the materials for grids, the quantity of antimony in an lead-antimony alloy is reduced in order to minimize the self-discharge and to facilitate the maintenance. Furthermore lead alloyed with calcium instead of antimony is used, and some lead acid batteries with the flat-plate type pasted positive plates made of a lead-calcium alloy have been already available in the market.
As to the bonding of the paste to the grids made of such a lead-calcium alloy, it has been experimentally confirmed that the bonding strength is not sufficient and must be improved.
As to the energy storage density, the grid serves not only for holding the paste in its pockets but also for conducting a current. Although the grid is not directly involved in the electrochemical reaction, it occupies a considerable part of the weight of the battery, which is one of the disadvantages of the grids. From the standpoint of productivity, one grid is used for one positive plate so that the grid type pasted positive plates are not adapted for the continuous mass production. In order to attain continuous mass production, the expanded metal type plates which are widely used in the alkaline storage batteries have been tried. The inexpensive lead acid batteries will be widely and mainly used for years for motive-power service such as electric cars, but they have their inherent problems in that the energy storage density is low and the life is short. In order to solve these problems, extensive studies and experiments have been conducted in various fields.
In order to improve the energy storage density, first the efficiency of an active material must be improved and the voltage must be increased, but these improvements are limited. When the efficiency is improved, the battery characteristics are improved in the initial service stage, but the life becomes shorter. Another countermeasure is to increase the ratio of the volume of an active material to the volume of a pasted positive plate; that is, to provide grids which are light in weight and small in size.
To this end, expanded metal, metal meshes and perforated metal sheets may be advantageously used, but the reduction in ratio of the volume of a support to the volume of a pasted positive plate is not preferable because the life is adversely affected. In other words, in order to improve the life, the support must contain the active material in a three-dimensional manner. However, the grids capable of this are very complex in construction and the ratio of the volume of the grid to the overall volume of the positive plate is increased. As a result, such grids are not preferable from the standpoint of the improvement of the energy storage density.
The flat-plate type pasted positive plates may be fabricated in a relatively simple process and have good discharge characteristics. However, even with the improved active materials and the improved materials and constructions of grids, there arises another serious problem of the increased separation of the active material from the plate due to the repetitive cycles of discharge and charging with an increased difference between a discharged voltage and a charged voltage. As a consequence, the life in terms of the number of cycles of discharge and charging is still short.
As is well known in the art, at a positive plate, lead dioxide PbO.sub.2 is converted to lead sulfate while at a negative plate lead is converted into lead sulfate, on discharge. Since diluted sulfuric acid is directly involved in the chemical reactions, it must be sufficiently diffused into the plates on discharge. In view of this fact, the idea of forming a thin resin layer on the plate has been rejected because the resin layer prevents sufficient diffusion of diluted sulfuric acid into the plates so that the discharge reaction is retarded even though the resin layer will be effective for preventing the separation of the active material from the plates.